1. Field of the Invention
This invention relates to a semiconductor device to be mounted on a circuit board by flip chip bonding with that bump electrode interposed therebetween, which is made of a low-melting-point metal, and more particularly to a connecting electrode portion employed in the semiconductor device.
2. Description of the Related Art
In a semiconductor device of this type, a multi-level metal layer, called a "BLM (Ball Limited Metal) layer" or a "barrier metal layer", is formed at a connecting electrode portion, and a bump electrode made of a low-melting-point metal such as solder is formed on the metal layer. A laminated structure consisting of two or three metal layers is known as a structure of the "barrier metal layer". In the case of the three-layer structure, a first-level metal layer is provided for enhancing the adhesion of the structure to an Al layer which is formed on a semiconductor chip to lead out an electrode. A second-level metal layer is provided for preventing diffusion of solder into the Al layer. A third-level metal layer is provided for protecting oxidation of the second-level metal layer.
As regards the materials of the barrier metal layer, Japanese Patent Application KOKOKU Publication No. 63-49377, for example, discloses a Ti metal layer as the first-level layer, an Ni metal layer as the second-level layer, and an SiO.sub.2 layer as an insulating layer between the Al layer and the multi-level metal layer.
In the general process for forming the above-described connecting electrode portion, the first-level metal layer or the first- through third-level metal layers are formed on the overall surface of an insulating layer which is formed on the Al layer and has an opening, and the resultant structure is patterned to desired shape and size. Thereafter, the bump electrode is formed on the barrier metal layer by electrolytic plating, etc.
The thus-formed connecting electrode portion, however, has various disadvantages as follows:
(1) Since the first-level metal layer is excessively oxidized in an etching process for patterning, the reliability of the connecting electrode portion is reduced. PA1 (2) In the case of using a Ti layer with oxygen dispersed therein as the first-level metal layer, an insulating layer made of a material other than SiO.sub.2 cannot have a sufficient adhesion to the first-level metal layer, and hence the use of such an insulating layer will cause a reduction in the reliability of the barrier metal layer after long use. PA1 (3) Although transition metals having high reactivity are generally used to form the first-level layer, their oxides are unstable and therefore reduce the reliability of the layer after connection. PA1 (4) Since an oxide in the first-level metal layer is an electrical insulator, the layer will show a high electrical resistance if it contains a great amount of oxygen.